Swallowing is a complex behavior in which the output of an integrative brainstem network gives rise to a patterned movement sequence described as the pharyngeal stage of swallowing. While several lines of evidence have demonstrated the importance of oropharyngeal sensory inputs in activating this medullary swallowing network, the range of afferent patterns that are both necessary and sufficient to evoke swallowing has not been fully elucidated. Stimulation of receptive fields innervated by the superior laryngeal nerve (SLN) or the pharyngeal branch of the glossopharyngeal nerve (GPNph) appear to be particularly effective in evoking or modulating the pharyngeal swallow; these “reflexogenic” areas correspond to the laryngeal mucosa, including the epiglottis and arytenoids, the lateral pharyngeal wall, posterior tonsillar pillar and peritonsillar areas.
In humans, the anterior faucial pillar historically has been considered the most reflexogenic site for swallowing. However, the recent finding that the pharyngeal swallow may begin after the bolus head passes the anterior faucial pillars in healthy adults, including geriatric adults, suggests that stimulation of more posterior pharyngeal regions may help facilitate the initiation of swallowing. The importance of more posterior oropharyngeal areas in swallowing elicitation is also suggested by anatomic evidence that the human posterior tonsillar pillar, as well as discrete regions of the palate, pharynx and epiglottis are innervated by a dense plexus formed from the GPNph and the internal branch of the SLN. The spatial correspondence between these areas of dual SLN/GPNph innervation and reflexogenic areas for swallowing has lead to the hypothesis that swallowing is elicited most readily by stimulation of areas innervated by both the GPNph and SLN. Dynamic stimuli that excite primary afferents within a number of receptive fields over time appear to elicit swallowing more readily than do static stimuli.
A variety of stimulus modalities have been applied in attempts to evoke swallowing (for review, see Miller, 1999). Repetitive electrical stimulation of the SLN or the GPN, particularly at stimulation frequencies between 30 and 50 Hz, evokes swallowing in a number of animal species. This suggests that the repetitive nature of the stimulus, and the repetition rate, are critical variables in swallowing elicitation. More recently, electrical stimulation of the pharynx has been reported to increase both the excitability and size of the pharyngeal motor cortex representation in humans (14), and facilitate swallowing in dysphagic patients following stroke. Mechanical and chemical stimuli can evoke swallowing in animal species. In humans, reports of the effects of cold mechanical stimulation of the anterior tonsillar pillar have been variable, some authors reporting decreases in swallowing latency and increases in swallowing frequency (16), and others failing to find an effect of this type of stimulation on oropharyngeal bolus transit, esophageal coordination, or the temporal pattern of swallowing. Three studies have examined the effects of cold mechanical stimulation applied to the anterior tonsillar pillars in small samples of dysphagic stroke patients. They reported a short-term facilitation of swallowing, measured in terms of reduced delay of the pharyngeal swallow, in some patients, with no related reduction in aspiration. Longitudinal studies, examining the potential long-term effects of oropharyngeal sensitisation on not only swallowing physiology but also on nutritional and respiratory health, have not been reported. Reports on the effects of gustatory stimuli also have been variable. A sour bolus has been reported to facilitate swallowing in stroke. Whereas some authors have reported that swallowing latency is significantly reduced by a combination of mechanical, cold, and gustatory (sour) stimulation, others have reported that a cold plus sour bolus reduces the speed of swallowing.
Air-pulse trains also have been considered as a stimulus that may faciliate the pharyngeal swallow. For example, a single air pulse is a dynamic stimulus that could be applied to a number of receptive fields including regions innervated by both the GPNph and SLN. Furthermore, an air-pulse train represents a repetitive stimulus that can be applied at specific frequencies and pressures. Some devices have been suggested for delivering such air-pulse trains, as disclosed for example in US patent application 2010/0016908, the entire disclosure of which is hereby incorporated herein by reference. The air pulse trains are directed to the oral cavity by way of an oral device, which is positioned and secured through various devices. For example, the '908 publication describes, in one embodiment, an “over-the-ear” oral device configured such that the flexible tubing that delivers the air pulse trains wraps around the ears of the user.